Apparatus and method for relieving dyspnoea

ABSTRACT

Apparatus ( 1 ) for relieving dyspnoea in an ambulatory exercising subject comprises a housing ( 3 ) to be worn on the subject, and within which a variable speed air blower motor ( 5 ) is located for delivering an air supply to the subject through a mouthpiece ( 10 ) at a pressure similar to average intrinsic positive end-expiratory pressure of the subject. A pressure transducer ( 25 ) in the mouthpiece ( 10 ) monitors the pressure during breathing cycles of the subject, and a control circuit ( 22 ) determines the average intrinsic positive end-expiratory pressure of the subject over five breathing cycles. The control circuit ( 22 ) operates the air blower motor ( 5 ) for delivering the air supply to the subject at a pressure similar to the determined average intrinsic positive end-expiratory pressure.

This application is a continuation of U.S. patent application Ser. No.09/831,388, filed Aug. 10, 2001, entitled “Apparatus and a Method forRelieving Dyspnoea,” which was a 35 U.S.C. § 371 national phase ofPCT/IE99/00112, filed on Nov. 8, 1999, which claims priority to IrishApplication Serial No. S980914, filed Nov. 6, 1998.

The present invention relates to apparatus and method for relievingdyspnoea, and in particular, though not limited to apparatus and methodfor relieving dyspnoea in an ambulatory subject, and more particularly,through not limited to an exercising ambulatory subject.

Subjects who suffer from respiratory disorders such as chronicobstructive pulmonary disease often suffer from dyspnoea orbreathlessness. This is particularly so in exercising subjects. The term“exercising” is used in this specification, in general, to includeactivities beyond the resting state, for example, the act of walking,ascending a stairs, or the like. Dyspnoea and the fear of a spell ofdyspnoea create great anxiety in such subjects, and can severely limittheir ability to perform even the most simple tasks, such as walking arelatively short distance, or ascending a stairs within a house.Apparatus for relieving dyspnoea in ambulatory subjects are known,although they suffer from limitations. Typically, such apparatus areportable and may be worn or carried by the subject. Typically, theyprovide a supply of air to a mouthpiece, a nasal mask or a face mask tobe worn by the subject, and the air pressure is at a pressure greaterthan ambient pressure. In general, a housing which houses a blower motoris worn or carried by the subject, and a communicating conduit deliversair from the blower motor to the mouth piece or mask at the pressuregreater than ambient. Typically, a controller is provided within thehousing for controlling the air blower for varying the pressure at whichthe air is supplied to the mouthpiece for the subject. A control knob onthe housing facilitates selection by the subject of the pressure atwhich the air is to be supplied, similarly controlled non-portableapparatus are also known.

While such apparatus are satisfactory for a subject who, for example, isresting, or alternatively is exercising at a relatively constant rateby, for example, walking at a constant rate, they are unsatisfactorywhere the rate of exercising by the subject is varying, for example, inthe case of a subject walking from a downstairs room to an upstairs roomin a house. While the subject is walking on level ground the exerciserate is at one level, however, when ascending the stairs, the exerciserate significantly increases. As the exercise rate increases the subjectbreathes faster. Faster breathing shortens the breathing cycle, and inparticular, the expiratory part of the breathing cycle. By shorteningthe expiratory part of the breathing cycle less time is available forthe lungs to empty. Since generally, ones lungs empty exponentially theshortened expiratory part of the cycle leads to a higher intrinsicpositive end-expiratory pressure which is the pressure in the breathingcycle at which expiration ends and the subject commences the inspiratorypart of the breathing cycle. Breathing being under the control of thenervous system is relatively oblivious to the mechanics of the operationof the lungs and their state of emptiness during a breathing cycle.Thus, as the intrinsic positive end-expiratory pressure increases over anumber of breathing cycles as a result of increase in the exercise rate,known apparatus are unable to cope with the increased exercise rate.Similar problems may arise in the case of a resting subject should anevent or crisis occur which causes an increase in the breathing rate ofthe subject.

There is therefore a need for apparatus and a method for relievingdyspnoea in a subject which overcomes these problems.

The present invention is directed towards providing such a method andapparatus.

According to the invention there is provided apparatus for relievingdyspnoea in a subject, the apparatus comprising an air supply means forproviding an air supply at a pressure greater than ambient for deliveryto the subject, wherein the apparatus further comprises a monitoringmeans for monitoring at least a part of at least one breathing cycle ofthe subject for determining the intrinsic positive end-expiratorypressure of a breathing cycle of the subject, and a control meansresponsive to the monitoring means for controlling the pressure of theair supply delivered to the subject at a pressure substantially matchedto the intrinsic positive end-expiratory pressure.

In one embodiment of the invention the control means controls thepressure of the air supply delivered to the subject at the pressuresubstantially matched to the intrinsic positive end-expiratory pressurefor a period at least at the end of the expiratory part of eachbreathing cycle.

In another embodiment of the invention the control means controls thepressure of the air supply delivered to the subject at a pressuregreater than the intrinsic positive end-expiratory pressure during atleast a part of the inspiratory part of each breathing cycle.

In a further embodiment of the invention the control means controls thepressure of the air supply delivered to the subject at the pressuresubstantially matched to the intrinsic positive end-expiratory pressureduring the expiratory part of each breathing cycle.

In a still further embodiment of the invention the control meanscontrols the pressure of the air supply delivered to the subject at apressure greater than the intrinsic positive end-expiratory pressureduring the inspiratory part of each breathing cycle.

In one embodiment of the invention the monitoring means monitors aplurality of breathing cycles, and a computing means is provided fordetermining the average intrinsic positive end-expiratory pressure overthe said plurality of breathing cycles.

In another embodiment of the invention the monitoring means monitorseach breathing cycle over the complete breathing cycle.

Preferably, the monitoring means is adapted for locating adjacent themouth of the subject.

In one embodiment of the invention the monitoring means is adapted forlocating in a mouthpiece, a nasal mask or a face mask or adjacentthereto through which the air supply is delivered to the subject.

In another embodiment of the invention signals are relayed from themonitoring means to the control means.

In one embodiment of the invention the monitoring means is connected tothe control means by hard wiring.

Alternatively, a means for transmitting an airborne signal from themonitoring means to the control means is provided, and a receiving meansis provided in the control means for receiving the airborne signaltransmitted from the monitoring means.

Preferably, the monitoring means is a pressure transducer for monitoringthe pressure of air during the breathing cycles.

Advantageously, the apparatus comprises a mouthpiece, nasal mask or aface mask, and a communicating means for communicating the mouthpiece ormask with the air supply means.

In one embodiment of the invention an exhaust means is provided in themouthpiece or mask for exhausting exhaled air from the subject, andpreferably, a valving means is provided in the exhaust means of themouthpiece or mask, the valving means being operable under the controlof the control means in response to the monitoring means for controllingthe pressure of the air supply in the mouthpiece.

In a still further embodiment of the invention the valving means is apressure regulating valving means.

In another embodiment of the invention the air supply means is operableunder the control of the control means in response to the monitoringmeans for controlling the pressure of the air supply delivered to thesubject.

In one embodiment of the invention the apparatus is portable and isadapted for use by an ambulatory exercising subject.

Ideally, the apparatus comprises a housing defining a hollow interiorregion, the air supply means being located within the hollow interiorregion. Preferably, the control means is located within the hollowinterior region of the housing. Advantageously, a power source islocated in the hollow interior region.

In one embodiment of the invention an air inlet and an air outlet areprovided to and from the hollow interior region and the air supply meansis located intermediate the air inlet and the air outlet.

Preferably, a first air filtering means is located adjacent the airinlet upstream of the air supply means, and preferably, a second airfiltering means is located adjacent the air outlet of the air supplymeans.

Advantageously, the air outlet is adapted for receiving thecommunicating means.

In one embodiment of the invention a strap is provided for securing thehousing to the subject.

In another embodiment of the invention a retaining means is provided onthe strap for retaining the mouthpiece or mask releasably secured to thestrap when not required.

In a further embodiment of the invention a receiving means is providedon the strap for receiving the communicating means. Preferably, thereceiving means is a releasable receiving means for releasably receivingthe communicating means.

In a further embodiment of the invention the communicating meanscomprises an elongated communicating conduit.

Advantageously, the communicating means comprises an elongated flexiblecommunicating conduit.

Ideally, the communicating conduit is of concertina type constructionfor facilitating flexing of the conduit and storing of the conduit.

In another embodiment of the invention the control means comprises thecomputing means.

In a further embodiment of the invention the air supply means comprisesan air blower motor. Preferably, the air blower motor is electricallypowered. Advantageously, the air blower motor comprises an impellerdriven by a motor.

In one embodiment of the invention the pressure at which the air supplyis delivered to the subject when the pressure of the air supply ismatched to the intrinsic positive end-expiratory pressure issubstantially similar to the intrinsic positive end-expiratory pressureof the subject monitored by the monitoring means.

Additionally the invention provides a method for relieving dyspnoea in asubject, the method comprising the steps of providing an air supplymeans for delivering an air supply to the subject at a pressure greaterthan ambient, wherein the method further comprises monitoring at least apart of at least one breathing cycle of the subject for determining theintrinsic positive end-expiratory pressure of the subject, andcontrolling the pressure of the air supply to the subject so that thepressure of the air supply delivered to the subject is substantiallymatched to the intrinsic positive end-expiratory pressure.

In one embodiment of the invention the pressure of the air supplydelivered to the subject is controlled at the pressure substantiallymatched to the intrinsic positive end-expiratory pressure for a periodat least at the end of the expiratory part of each breathing cycle.

In another embodiment of the invention the pressure of the air supplydelivered to the subject is controlled at a pressure greater than theintrinsic positive end-expiratory pressure during at least a part of theinspiratory part of each breathing cycle.

In a further embodiment of the invention the pressure of the air supplydelivered to the subject is controlled at the pressure substantiallymatched to the intrinsic positive end-expiratory pressure during theexpiratory part of each breathing cycle.

In a still further embodiment of the invention the pressure of the airsupply delivered to the subject is controlled at a pressure greater thanthe intrinsic positive end-expiratory pressure during the inspiratorypart of each breathing cycle.

In one embodiment of the invention a plurality of breathing cycles ofthe subject are monitored for determining the average intrinsic positiveend-expiratory pressure over the said plurality of breathing cycles.

In another embodiment of the invention the complete breathing cycle ofeach monitored breathing cycle is monitored.

Preferably, the air pressure adjacent the mouth of the subject ismonitored for determining the intrinsic positive end-expiratorypressure.

Advantageously, the air pressure is monitored in a mouthpiece, nasalmask or a face mask attached to the subject for determining theintrinsic positive end-expiratory pressure.

In one embodiment of the invention the pressure at which the air supplyis delivered to the subject is at a pressure substantially similar tothe determined intrinsic positive end-expiratory pressure.

In another embodiment of the invention the method is for relievingdyspnoea in an ambulatory exercising subject, and the method comprisesthe step of providing the air supply means as a portable air supplymeans.

The advantages of the invention are many. By virtue of the fact that theapparatus and method control the pressure at which the air supply issupplied to the subject at the end of the expiratory part of eachbreathing cycle so that the pressure of the air supply to the subject ismatched to the intrinsic positive end-expiratory pressure of thesubject, the subject can effectively commence inspiration immediatelythe expiratory part of the breathing cycle has ended. This permitsmaximum intake of air by the subject during the inspiration part of eachbreathing cycle. In other words, by virtue of the fact that the subjectcan commence inspiration immediately at the end of the expiratory partof the breathing cycle air can be drawn into the lungs of the subjectover the entire inspiratory part of the breathing cycle. This, thus,maximises the volume of air drawn into the lungs of the subject duringeach breathing cycle. The matching of the pressure of the air supplydelivered to the subject with the intrinsic positive end-expiratorypressure of the subject at the end of the expiratory part of eachbreathing cycle removes the threshold load which subjects typicallyencounter at the commencement of the inspiratory part of a breathingcycle. This threshold load which would otherwise be encountered by asubject is caused where the pressure of the air supply delivered to thesubject is below the intrinsic positive end-expiratory pressure. Thus,by providing the air supply to the subject at a pressure matched to theintrinsic positive end-expiratory pressure at the end of the expiratorypart of each breathing cycle the threshold load is eliminated.

In embodiments of the invention where the pressure of the air supply tothe subject during part or all of the inspiratory part of each breathingcycle is greater than the intrinsic positive end-expiratory pressure, afurther advantage is achieved in that a greater volume of air can bedrawn into the lungs of the subject during the inspiratory part of thecycle.

The invention will be more clearly understood from the followingdescription of some preferred embodiments thereof which are given by wayof example only with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of apparatus according to the invention forrelieving dyspnoea in an ambulatory exercising subject,

FIG. 2 is a perspective view of the apparatus of FIG. 1 in use,

FIG. 3 is a block representation of a circuit of the apparatus of FIG.1,

FIG. 4 illustrates graphs of a number of consecutive breathing cycles ofan ambulatory subject during exercising,

FIG. 5 is a view similar to FIG. 3 of apparatus according to anotherembodiment of the invention for relieving dyspnoea in an ambulatoryexercising subject, and

FIG. 6 is a side elevational view of a portion of the apparatus of FIG.5.

Referring to the drawings and initially to FIGS. 1 to 4 there isillustrated apparatus according to the invention indicated generally bythe reference numeral 1 for relieving dyspnoea in a subject, and in thisembodiment of the invention in an ambulatory exercising subject. Theapparatus 1 is portable and is suitable for wearing by the subject asillustrated in FIG. 2. The apparatus 1 comprises a housing 3 whichdefines a hollow interior region 4. An air supply means, namely, avariable speed air blower motor 5 having an impeller 6 is located withinthe hollow interior region 4 for delivering an air supply to the subjectat a pressure greater than ambient pressure as will be described below.An air inlet port 7 in the housing 3 accommodates air into the hollowinterior region 4 to be blown by the air blower motor 5 and the impeller6 through an outlet port 8 for delivery to the subject.

A communicating means, namely, a communicating conduit 9 of flexibleplastics material communicates the outlet port 8 with a mouthpiece 10 towhich the air supply is delivered to the subject. The communicatingconduit 9 is of concertina construction for facilitating flexing of theconduit 9 and storing thereof. An exhaust means, namely, an exhaust port11 in the mouthpiece 10 assists in exhausting expired air from thesubject. A first air filtering means provided by a first air filter 12located in the air inlet port 7 filters air to the air blower motor andthe impeller 6, while a second air filtering means, namely, a second airfilter 13 is located in the air outlet port 8 for filtering the airsupply to the subject, and also for filtering expired air which may flowback through the conduit 9. A waist strap 15 and a shoulder strap 16secured to the housing 3 releasably secure the housing 3 to the subject.A retaining means, namely, a retaining clip 17 is provided on theshoulder strap 16 for releasably retaining the mouthpiece 10 on theshoulder strap 16 when not in use. A receiving means, namely, areceiving clip 18 on the shoulder strap 16 releasably receives andsecures the communicating conduit 9 to the shoulder strap 16.

A power supply means, namely, a DC power source provided by a battery 20is located in the housing 3 for powering the air blower motor 5. Thespeed of the blower motor 5 is controlled by a control means, namely, acontrol circuit 22 for varying the flow rate of the air supply to thesubject, for in turn varying the pressure at which the air supply isdelivered to the subject.

A monitoring means, in this embodiment of the invention provided by apressure transducer 25 is located in the mouth piece 10 for monitoringair pressure in the mouth piece 10 during a plurality of completebreathing cycles so that the average intrinsic positive end-expiratorypressure of the subject may be determined.

Referring in particular to FIG. 4, the intrinsic pressure and flow rateof air during the inspiratory part and the expiratory part of abreathing cycle of an ambulatory exercising subject during ten typicalbreathing cycles is illustrated. It should be noted that the graphs Aand B are not to scale, and are provided solely for the purpose ofillustrating typical breathing cycles. Graph A illustrates the intrinsicpressure, while graph B illustrates the intrinsic flow rate of air.During the first three breathing cycles the subject is exercising at arelatively low constant rate, for example, walking. However, during thenext seven breathing cycles the exercise rate of the subject isincreased, for example, by changing from walking to running, orascending a stairs or the like. As can be seen from graph A as theexercise rate increases the breathing cycle time is reduced. In otherwords, the breathing cycle time for cycles four to ten is less than thebreathing cycle time for the first three breathing cycles. Inparticular, as the breathing cycle time is reduced the time period ofthe expiratory part of each breathing cycle from the positive peak X tothe point Y in each breathing cycle of graph A is likewise reduced. Ascan be seen from graph B this, thus, reduces the volume of air which canbe exhaled by the subject, and in turn raises the intrinsic positiveend-expiratory pressure of each breathing cycle. The intrinsic positiveend-expiratory pressure of each breathing cycle is the point Y where thegraph Y cuts the verticals C. In the first three breathing cycles thepoint Y lies on the x-axis. Thus, in the first three breathing cyclesthe subject can commence drawing air into his or her lungs at the verybeginning of the inspiratory part of each breathing cycle, in otherwords, at the point Y. However, as the exercise rate increases the pointY rises above the x-axis, and thereby, although expiration has ceased atthe point Y, inspiration cannot commence until the intrinsic pressurehas dropped to the x-axis, in other words, until the point Z has beenreached. Therefore, the subject is unable to draw air into his or herlungs during the period Y to Z of each breathing cycle, in other words,during the time period between the adjacent verticals C and D.

The apparatus as will be described below by monitoring the intrinsicpressure of the breathing cycles of the subject, and by determining theaverage intrinsic positive end-expiratory pressure, in other words, theaverage of the pressure values of the points Y over a plurality ofbreathing cycles, in this case, five breathing cycles, and then byincreasing the pressure of the air supply to the subject to the averageof the pressure values of the points Y, allows the subject to commencedrawing in air immediately at the commencement of the inspiratory partof each breathing cycle. In other words, the subject can draw air intohis or her lungs immediately the point Y has been reached in eachbreathing cycle.

The pressure transducer 25 monitors air pressure and may also monitorair flow in the mouthpiece 10. Signals from the pressure transducer 25are relayed through hard wiring 27 to the control circuit 22. Acomputing means provided by a microprocessor 28 in the control circuit22 determines the average intrinsic positive end-expiratory pressure ofthe subject from the signals received from the pressure transducer 25using one or more appropriate algorithms. Typically, signals from fiveconsecutive breathing cycles are read by the microprocessor 28, and theaverage intrinsic positive end-expiratory pressure of the subject isdetermined over the five breathing cycles. The determination ofintrinsic positive end-expiratory pressure in a subject will be wellknown to those skilled in the art, and it is not intended to describethe algorithms in detail. The microprocessor 28 in the control circuit22 controls the control circuit 22 for in turn controlling the airblower motor 5 for delivering the air supply at an appropriate flow rateto the mouthpiece 10 so that the air supply delivered to the mouthpiece10 is at a pressure matched to the determined average intrinsic positiveend-expiratory pressure. In this embodiment of the invention the airsupply is delivered to the mouthpiece 10 at a pressure similar to thedetermined average intrinsic positive end-expiratory pressure. Thecontrol circuit 22 controls the air blower motor 5 for maintaining thepressure of the air supply at the appropriate pressure until signalsreceived from the pressure transducer 25 in the mouthpiece 10 indicatethat a further change in the average intrinsic positive end-expiratorypressure has occurred. At which stage the control circuit 22 againcontrols the blower motor 5 to again match the pressure of the airsupply to the newly determined average intrinsic positive end-expiratorypressure.

A control knob 30 located exteriorly on the housing 3 is operablyconnected to the control circuit 22 for providing manual control of thecontrol circuit 22 for in turn manually controlling the pressure atwhich the air blower motor 5 delivers the air supply to the mouth piece10, should the subject decide to control the air supply pressuremanually. A button switch 32 also located exteriorly on the housing 3 isoperably connected to the control circuit 2 for facilitating selectionby the subject between an automatic operation mode of the apparatus 1and a manual operation mode of the apparatus 1. When the apparatus 1 isoperated in the manual mode the pressure at which the air supply isdelivered to the mouth piece 10 is controlled by the control knob 30,and when the apparatus 1 is operated in the automatic mode the pressureat which the air supply is delivered to the mouth piece 10 is determinedby the control circuit 22 from signals received from the pressuretransducer 25 in the mouth piece 10. An indicator light 33 on thehousing 3 indicates when the apparatus 1 is operating in the automaticmode.

In use, a subject wears the apparatus 1 as illustrated in FIG. 2, andwhen a supply of air at a pressure greater than ambient is required thesubject breaths through the mouth piece 10. If it is desired to operatethe apparatus 1 in the manual operating mode, the button switch 32 isdepressed and the control knob 30 is rotated until the air supply to thesubject is at the desired pressure. When it is desired to operate theapparatus 1 in the automatic operating mode the button switch 32 isagain depressed switching the apparatus 1 to operate in the automaticmode. In this mode, signals from the pressure transducer 25 are read bythe microprocessor 28 in the control circuit 22 which continuouslydetermines the average intrinsic positive end-expiratory pressure. Thecontrol circuit 22 sets the speed of the blower motor 5 at anappropriate speed to deliver the air supply at a flow rate so that thepressure of the air supply delivered to the mouthpiece 10 is similar tothe determined average intrinsic positive end-expiratory pressure. Thepressure transducer 25 continues to monitor the intrinsic pressure ofthe breathing cycles of the subject, and the microprocessor 28continuously determines the average intrinsic positive end-expiratorypressure from the signals received from the pressure transducer 25. On achange in the average intrinsic positive end-expiratory pressure beingdetermined the control circuit 22 resets the speed of the blower motor 5for delivering the air supply at the appropriate flow rate so that thepressure of the air supply in the mouthpiece 10 is matched to the newaverage intrinsic positive end-expiratory pressure, and so operation ofthe apparatus 1 continues.

Referring now to FIGS. 5 and 6 there is illustrated apparatus accordingto another embodiment of the invention indicated generally by thereference numeral 40 also for relieving dyspnoea in an ambulatoryexercising subject. The apparatus 40 is substantially similar to theapparatus 1 and similar components are identified by the same referencenumerals. The main difference between the apparatus 40 and the apparatus1 is that the blower motor 5 is a constant speed motor, and thus,delivers the air supply at a constant pressure. In this embodiment ofthe invention the air supply is provided to the subject through a mouthand nasal mask 41 within which the pressure transducer 25 is located. Anexhaust means provided by an exhaust port 42 from the mouth and nasalmask 41 exhausts expired air from the subject. A valving means, in thisembodiment of the invention a variable pressure regulating valve 44 islocated in the exhaust port 42 for controlling the pressure of the airsupply in the mouth and nasal mask 41. The valve 44 is hard wired to thecontrol circuit 22 by a cable 45, and the control circuit 22 controlsthe valve 44 for maintaining the pressure in the mouth and nasal mask 41at a pressure similar to the average intrinsic positive end-expiratorypressure determined by the microprocessor 28 in response to signalsreceived from the pressure transducer 25. In this embodiment of theinvention the only control exercised over the air blower motor 5 by thecontrol circuit 22 is to switch on and off the air blower motor 5 as theapparatus 40 is required to deliver an air supply to the subject.

Operation of the apparatus 40 is substantially similar to that of theapparatus 1. The average intrinsic positive end-expiratory pressure ofthe subject is determined by the microprocessor 28 of the controlcircuit 22 from signals read from the pressure transducer 25. Ondetermining the average intrinsic positive end-expiratory pressure, thecontrol circuit 22 then sets the valve 44 appropriately so that the airsupply in the mouth and nasal mask 41 is at a pressure similar to theaverage intrinsic positive end-expiratory pressure determined by themicroprocessor 28. On the microprocessor 28 determining a change in theaverage intrinsic positive end-expiratory pressure, the control circuit22 resets the valve 44 appropriately, so that the air supply in the mask41 is again at a pressure similar to the new average intrinsic positiveend-expiratory pressure, and so operation of the apparatus 40 continues.Otherwise, operation of the apparatus 40 is similar to that of theapparatus 1.

It is envisaged that the microprocessor 28 of the apparatus 1 and theapparatus 40 may be programmed so that the air supply is delivered tothe subject at a pressure similar to the average intrinsic positiveend-expiratory pressure during the expiratory part of each breathingcycle, and just after the commencement of the inspiratory part of eachbreathing cycle, the pressure of the air supply to the subject would beraised to a pressure greater than the average intrinsic positiveend-expiratory pressure, and the pressure of the air supply to thesubject would be maintained at that greater pressure until the end ofthe inspiratory part of each breathing cycle, at which stage thepressure of the air supply would be reduced to the average intrinsicpositive end-expiratory pressure. This would facilitate inspiration bythe subject so that the subject could draw in a greater volume of airduring the inspiratory part of the breathing cycle. At the end of theinspiratory part of each breathing cycle the air blower motor or thevalve 44 would be operated to deliver the air supply to the subject at apressure similar to the average intrinsic positive end-expiratorypressure until the expiratory part of each breathing cycle had beencompleted. Should the microprocessor be programmed in this way themicroprocessor would determine the beginning and end of the respectiveexpiratory and inspiratory parts of the breathing cycle by reading thesignals from the pressure transducer 25.

It is envisaged that an oxygen supply may be provided in the housing formixing with the air supply being delivered to the subject. In the eventof an oxygen supply being provided, the control circuit would controlthe supply of oxygen into the air supply so that the pressure at whichthe oxygen is being delivered to the subject is similar to pressure atwhich the air supply is being delivered.

While the air supply is described as being delivered to the subjectthrough a mouthpiece or mouth and nasal mask, the air supply may bedelivered to the subject through any other suitable means, for example,a nasal mask, a face mask or the like.

It will be appreciated that while the air supply means has beendescribed as comprising a blower motor and impeller, any other suitableair supply means may be provided, for example, an air compressor, adiaphragm pump, a piston pump of an appropriate size and with anadequate dynamic response.

While the pressure transducer has been described as being hard wired tothe control circuit, it will be appreciated that signals from thepressure transducer may be relayed to the control circuit by radio wavesfrom a radio transmitter located adjacent the pressure transducer to aradio receiver located in the control circuit. Needless to say, anyother airborne communicating waves may be used for relaying signals fromthe pressure transducer to the control circuit.

While the monitoring means has been described as being a pressuretransducer located adjacent the mouth of the subject, the monitoringmeans may be provided by a pressure transducer located in the housingdownstream of the air supply means. In which case a flow meter would beprovided for measuring the flow rate of the air supply from the airsupply means, and knowing the resistance of the communicating means theintrinsic pressure during each breathing cycle of the subject could bedetermined from signals read from the pressure transducer and the flowmeter.

While the average intrinsic positive end-expiratory pressure has beendescribed as being determined over five breathing cycles, the averageintrinsic positive end-expiratory pressure may be determined over anyother suitable number of breathing cycles. Furthermore, in certain casesit is envisaged that instead of determining the average intrinsicpositive end-expiratory pressure, intrinsic positive end-expiratorypressure may be used directly.

1-16. (Cancelled).
 17. An apparatus for relieving dyspnoea in a subject, the apparatus comprising: a gas flow generating system adapted to provide a flow of gas; monitoring means for monitoring a characteristic associated with a breathing cycle of the subject; controlling means for determining an average intrinsic positive end-expiratory pressure based on an output of the monitoring means, and for controlling the gas flow generating system such that a pressure of the flow of gas delivered to the subject during at least a portion of an expiratory phase of a breathing cycle substantially corresponds to the average intrinsic positive end-expiratory pressure.
 18. The apparatus as claimed in claim 17, wherein the controlling means controls the gas flow generating system such that the pressure of the flow of gas delivered to the subject during at least a portion of an inspiratory phase of a breathing cycle is at a pressure greater than the average intrinsic positive end-expiratory pressure.
 19. The apparatus as claimed in claim 17, wherein the gas flow generating system includes a blower motor, and wherein the controlling means controls the pressure provided by that gas flow generating system by controlling an operating speed of the blower motor.
 20. The apparatus as claimed in claim 17, wherein the monitoring means is located proximate to an airway of the subject.
 21. The apparatus as claimed in claim 20, further comprising: a patient circuit having a first end operatively connected to the gas flow generating system and a second end; and a patient interface operatively connected to the second end of the patient circuit, and wherein the monitoring means is operatively connected to the patient interface.
 22. The apparatus as claimed in claim 17, wherein the monitoring means is connected to the controlling means by a wire.
 23. The apparatus as claimed in claim 17, wherein the monitoring means includes means for transmitting a wireless signal to the controlling means, and wherein the controlling means includes receiving means for receiving the wireless signal.
 24. The apparatus as claimed in claim 17, wherein the monitoring means is a pressure transducer.
 25. The apparatus as claimed in claim 17, wherein the apparatus is portable and is adapted for use by an ambulatory subject.
 26. The apparatus as claimed in claim 17, wherein the gas flow generating system comprises an electrically powered blower motor.
 27. Apparatus for relieving dyspnoea in a subject, the apparatus comprising: a gas flow generating system adapted to provide a flow of gas, monitoring means for monitoring a characteristic associated with a breathing cycle of a subject; controlling means for controlling the pressure of the flow of gas provided by the gas flow generating system; a patient circuit having a first end operatively connected to the gas flow generating system and a second end; a patient interface operatively connected to the second end of the patient circuit; and exhausting means operatively coupled to the patient interface for exhausting exhaled gas from the subject, wherein the exhausting means includes a valve operable under control of the controlling means for controlling a pressure of the flow of gas in the patient interface.
 28. The apparatus as claimed in claim 27, wherein the valve is a pressure regulating valve.
 29. The apparatus as claimed in claim 27, wherein the gas flow generating system includes a blower motor, and wherein the controlling means controls the pressure provided by that gas flow generating system by controlling an operating speed of the blower motor.
 30. A method for relieving dyspnoea in a subject, the method comprising the steps of: delivering a flow of gas to an airway of a subject at a pressure greater than ambient; determining an intrinsic positive end-expiratory pressure of the subject; and controlling the pressure of the flow of gas delivered to the subject during an expiratory phase of a breathing cycle such that the pressure of the flow of gas substantially corresponds to an average intrinsic positive end-expiratory pressure. 